For radio communication, the non-linearity of a power amplifier included in a transceiver is one of the dominate impairments in wireless systems. Due to the nature of the dynamic range of transmit and receive radio signals, it is sometimes necessary to back-off or reduce gain in a transmit or receive signal path in order to reduce the signal distortion. Such gain reduction can have significant impact on system performance and power efficiency.
Recently, Orthogonal Frequency Division Multiplexing (OFDM), which is one standard multi-carrier modulation scheme, has been used in IEEE802.11.a, Hiperland/2. One advantage in selecting the OFDM as the standard modulation scheme is realized by the capability to combat frequency selective fading, its spectrum efficiency and its low implementation complexity. However, one significant drawback of OFDM is its vulnerability to non-linear distortion due high peak to average ratio (PAR). Tremendous effort has been made to reduce the impact of non-linearity in radio receivers, which is caused by amplifiers included in transceivers of the radio receiver.
In a communication system, it is assumed that there is one wideband receiver and two users. Further, it is assumed that each of the two users are associated with a spectrum and that each spectrum associated with each of the two users are defined adjacent to each other. The signal received at the receiver due to the non-linear distortion is represented by:y(n)=f1(h1(n)*s1(n))+f2(h2(n)*s2(n))+v(n)  (1)where, h1(n) and h2(n) are the channel impulse response of two users without non-linear distortion, s1(n) and s2(n) are the modulated signals, f1(x) and f2(x) are the non-linear functions, and v(n) is the background noise.
There are two non-linear functions, which are widely used in the applications:
1. RappPa model:
                              d          ⁡                      (            x            )                          =                ⁢                              (                          1              +                                                                  x                                                                    2                  ⁢                  p                                                      )                                1                          2              ⁢              p                                                                        y          ⁡                      (            x            )                          =                ⁢                  x                      d            ⁡                          (              x              )                                          2. Polynomial model:y(x)=a1x+a3x3 
The impact of non-linear distortion on the signal spectrum is shown in FIG. 1, where in the RappPa model, p=2. In this case, the users wouldn't interfere with each other, since the signal powers of two users at the receiver are comparable. The SNR is large enough and any small leakage from an adjacent channel wouldn't cause any performance degradation.
As illustrated in FIG. 1, the adjacent interference is about 25 dB below the signal level. Further, the ACI has no impact on the performance of user 1 and user 2.
In typical application environments, however, different channel fading experienced by individual users and different distances of each user from the receiver can adversely affect the non-linear distortion on the signal spectrum, which is exemplified in FIG. 2. In FIG. 2, the receiver powers are not comparable in that the received power of user 2 has about 15 dB higher than that of user 1. In this case, the Adjacent Channel Interference (ACI) caused by user 2 may start to interfere with the signal associated user 1 at level of about 10 dB. This interference between the signals associated with user 1 and user 2 can undesirably cause Bit-Error-Rate (“BER”) performance degradation.
Non-linear distortion due to the non-linear amplifier located in the radio transmitter and receiver, as well as the high peak-to-average-power-ratio modulation, may cause the reduction of signal power in the in-band channel(s). The reduction of signal power in the in-band channel(s) may further increase the interference level in the out-of-band adjacent channel(s). This ACI is also called the side-lobe of the in-band signal, which can lead to severe performance degradation if the power level of the adjacent user is comparable to the power level of the ACI.
It would, therefore, be desirable to provide a method and apparatus for combatting non-linear distortion.